The exposure of an image detector such as an image sensor must be controlled to prevent overexposure, which causes clipping, and underexposure which results in excessive noise. Exposure control is generally accomplished by a shutter that blocks light when it is closed, and allows light to pass when it is open. Aperture control and neutral density filters can be used to reduce the intensity of light, and to allow the image detector to build up its exposure more gradually. However, virtually all cameras implement shutters.
In film cameras, the shutter is a mechanical mechanism. It can range in complexity from a simple spring-loaded pinhole punched in a dark plate to the complicated multiple-blade single-lens reflex (SLR) structure. These shutters are not fast enough, or reliable enough, to operate at high frame rates. Accordingly, electronic video cameras typically utilize electronic shutters and not mechanical shutters.
A number of different electronic shutters have been implemented in CCD and CMOS image sensors. CCD image sensors typically use a “global” shutter, meaning that all of the pixels in the image sensor are exposed to the image at the same time. The most common CCD shutter techniques are frame transfer (FT), interline transfer (IT), and a hybrid of both techniques known as frame interline transfer (FIT).
A basic full frame (FF) CCD image sensor 10 is illustrated in FIG. 1. The basic full frame CCD image sensor 10 has no shutter mechanism. Photocharge accumulates in photosensors (typically photodiodes) within vertical registers 12. The photocharge is transferred to a horizontal register 14 before being clocked out of the sensor 14 as an image output. Because this process operates at a slow rate (approximately 200 nsec/pixel or 50 microseconds per row), the accumulation of charge in the vertical register 12 causes vertical streaks to appear in the output image. The streaks are known in the art and are often referred to as “smear.”
A frame transfer CCD image sensor 30 is illustrated in FIG. 2. This sensor 30 includes an image area 32 comprising vertical registers 12 for capturing the image and a storage area 34 also comprising vertical registers 36 for storing the image captured in the image area 32. The use of the storage area 34 is designed to reduce smear. Photocharge is transferred from the image area 32 to the storage area 34 rapidly, and then read out from the storage area 34 at the normal rate. Smear can be substantially reduced, but cannot be entirely eliminated. In addition, this type of image sensor 30 is costly because the area of the sensor 30 is roughly doubled. Moreover, the image may also be corrupted by dark current while it is being stored. Dark current is the phenomenon whereby current is generated as a photodiode/photosensor signal even though there is an absence of light and no signal should be present.
An interline transfer CCD image sensor 50 is illustrated in FIG. 3. This sensor 50 collects photocharge in photodiodes 56 that are separate from the vertical registers 52. The vertical registers 52 are masked with metal 58 to prevent charge accumulation and reduce smear. The transfer of charge from the photodiodes 56 to the vertical registers 52 can be accomplished rapidly. However, it is burdensome and extremely difficult (if not impossible) to completely mask the vertical register 52 structures from light. Accordingly, some smear remains in the image output.
FIG. 4 illustrates a frame interline transfer (FIT) CCD image sensor 70, which is essentially a combination of the frame and interline transfer image sensors 30 (FIG. 2), 50 (FIG. 3). That is, the FIT CCD image sensor 70 includes an image area 72 and a storage area 74, like the frame transfer image sensor 30 (FIG. 2). The storage area 74 includes vertical registers 76 that output to a horizontal register 14 as described above. Similar to the interline transfer CCD image sensor 50 (FIG. 3), the image area 72 of the FIT CCD image sensor 70 uses photodiodes 56 that are separate from the vertical registers 52. In addition, the vertical registers 52 of the image area 72 and the vertical registers 76 of the storage area 74 are masked with metal 78 to prevent charge accumulation and to reduce smear. As can be appreciated, however, the frame interline transfer CCD image sensor 70 is relatively expensive and is generally used only in commercial broadcast cameras.
Currently, there is a movement towards using CMOS image sensors as low cost imaging devices. A CMOS image sensor includes a focal plane array of pixels, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge. Each pixel has a charge storage region, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some image sensor circuits, each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level.
In a CMOS image sensor, the active elements of a pixel perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the photosensor and storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and buffering of a signal representing pixel charge. Photocharge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by the source follower output transistor.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524 and 6,333,205, assigned to Micron Technology, Inc.
A typical four transistor (4T) CMOS image pixel 100 is shown in FIG. 5. The pixel 100 includes a photosensor 102, implemented as a pinned photodiode, transfer transistor 104, floating diffusion region FD, reset transistor 106, source follower transistor 108 and row select transistor 110. The photosensor 102 is connected to the floating diffusion region FD by the transfer transistor 104 when the transfer transistor 104 is activated by a transfer gate control signal TX. Capacitor 112 represents the floating diffusion region FD. Photosensor charge is converted to a voltage on this capacitor 112.
The reset transistor 106 is connected between the floating diffusion region FD and a pixel supply voltage Vpix. A reset control signal RST is used to activate the reset transistor 106, which resets the floating diffusion region FD to the pixel supply voltage Vpix level as is known in the art. The source follower transistor 108 has its gate connected to the floating diffusion region FD and is connected between the supply voltage Vpix and the row select transistor 110. The source follower transistor 108 buffers the floating diffusion region FD and keeps the charge from leaking out while transferring the electrical output signal to OUTPUT. The row select transistor 110 is controllable by a row select signal ROW for selectively connecting the source follower transistor 108 and the output voltage signal OUTPUT to a column line 114 of a pixel array.
Two different shutter methods can be implemented in CMOS image sensors to operate the pixel 100. In a global shutter operating methods, all of the photosensors 102 in a pixel array accumulate charge over the same time interval. In the global shutter method, the reset transistor 106 and transfer transistor 104 of all pixels 100 are operated simultaneously. The reset and transfer transistors 106, 104 are turned on (i.e., activated), initially, to reset all of the pixels 100 to Vpix. Charge integration (also known as the integration period) begins when the transfer and reset transistors 106, 104 are turned off (i.e., deactivated). At the end of the integration period, the transfer transistor is turned on (via the TX signal), and photocharge flows to the floating diffusion region FD. Typically, the floating diffusion region FD is masked by e.g., metal to limit the further accumulation of charge at the region FD. Photocharge remains on the floating diffusion region FD until it is read out by activating the row select transistor 110 (via the ROW signal) and output on the column line 114. Because it is difficult to keep stray light from reaching the floating diffusion region FD, the global shutter method of operating a CMOS image sensor also suffers from smear. As is known in the art, the CMOS image sensor also suffers from kT/C noise because correlated double sampling is not performed when using the global shutter mode of operation.
In the rolling shutter operational method/mode, the exposure interval varies from row to row. The first row in the array begins integrating first, and then the next rows sequentially begin integrating. The last row in the array will integrate last. The integration period is the same for all rows. The image is substantially free of image smear when the image sensor is operated in the rolling shutter mode. Moreover, kT/C noise may be eliminated because correlated double sampling may be used with a rolling shutter. If there is any motion in the scene, however, the varying integration interval causes motion artifacts to appear. The artifacts typically distort the shape of a moving object such as e.g., a square is often distorted into a parallelogram. The degree of distortion depends on the speed of the object relative to the readout speed of the image sensor; as can be appreciated, high readout speeds are necessary to minimize this affect.
CCD and CMOS image sensors are often used in digital single-lens reflex (DSLR) cameras. DSLR cameras have evolved from conventional film SLR cameras. In a DSLR camera, the film is replaced with a CCD or CMOS image sensor that sits in the film plane. Exposure is controlled by a fast mechanical multiple-blade focal-plane shutter 150, as shown in FIG. 6. The blades 152, 154 of the shutter 150 travel across the focal plane of the image sensor 160 at high speed (e.g., approximately 1/240th of a second). The blades 152, 154 travel in the direction of arrow 156. The imager sensor's 160 exposure time is determined by the gap 158 between the edges of the leading and trailing blades 152, 154, and the speed of the shutter 150. This type of exposure method is analogous to the rolling shutter mode of operation for a CMOS image sensor. In fact, the motion artifacts induced by the shutter method illustrated in FIG. 6 and the rolling shutter method are identical.
Accordingly, there is a need and desire for a CMOS sensor that captures images substantially free of smear and kT/C noise in which motion artifacts are negligible.